Electrochemical oxidation of thallium derivatives

ABSTRACT

A VOLATILE WEAK ACID, STAINLESS STEEL, LEAD OR COPPER ELECTRODES CAN BE SATISFACTORILY.   THALLOUS IONS ARE OXIDISED ELECTROCHEMICALLY TO THALLIC IONS IN A CELL DIVDED INTO AT LEAST TWO COMPARTMENTS BY AN ANION EXCHANGE MEMBRANE DIVIDER, THE ANOLYTE COMTAINING THALLOUS AND THALLIC IONS AND THE CTHOLYTE CONTAINING AN ALKALI METAL OR AMMONIUM HYDROXIDE OR SALT OF

ELECTROCHEMICAL OXIDATION OF THALLIUM DERIVATIVES Filed Feb. 16, 1971 II HHI' mm:

INVENTORS LOUIS LE BRIS, DANIEL MICHELET, MICHEL RAKOUTZ AttorneysUnited States Patent (ifice Patented Sept. 18, 1973 Int. Cl. C01g15/02); C23b 5/30, 5/46 US. Cl. 204-93 16 Claims ABSTRACT OF THEDISCLOSURE Thallous ions are oxidised electrochemically to thallic ionsin a cell divided into at least two compartments by an anion exchangemembrane divider, the anolyte containing thallous and thallic ions andthe catholyte containing an alkali metal or ammonium hydroxide or saltof a volatile weak acid. Stainless steel, lead or copper electrodes canbe used satisfactorily.

The present invention relates to an electrochemical process foroxidising thallous ions to thallic ions.

It has been known for a long time that thallous ions may be oxidisedelectrochemically to thallic ions, see G. Grube et al., Zeitsch. fiirElektrochem. 26, 291-7 (1920). The Grube et al. process used anelectrolysis cell divided into two compartments by means of a diaphragm.However, the diaphragms employed at that time were not selective withregard to the ions, and as a result, a substantial migration of thallicions towards the cathode compartment was produced, which resulted in apoor electric current yield. A11 electrochemical process for oxidisingthallous ions similar to that of Grube et a1 has been described in U.S.patent specification No. 3,479,262.

Another process for the electrochemical oxidation of a solution ofthallous or cerous ions is described in US. patent specification No.3,486,992, which involves passing the solution of ions to be oxidisedfirst into the cathode compartment and then into the anode compartmentof an electrolytic cell, the cathode consisting of a material of whichthe hydrogen overvoltage is sufiiciently low to prevent the reduction ofthe metal ions to the zero valency state (metal state).

When the metal ions reducible to metals are thallous and/or thallicions, scarcely anything other than platinised platinum or platinisedtitanium is suitable for the cathode. The more common and generally lessexpensive metals such as stainless steel or copper cannot be used inthis process.

The main subject of the present invention is to provide anelectrochemical process for oxidising thallous ions which does not havethe aforementioned disadvantage and which allows cathodes based oncommon metals to be employed.

The present invention provides a process for the electrochemicaloxidation of thallous ions to thalic ions which comprises passing adirect electric current between an anode and a cathode of anelectrolysis cell divided into at least two compartments by at least onemembrane having anion exchange properties, the anolyte comprisingthallous and thallic ions and the catholyte comprising an alkali metalhydroxide, an ammonium hydroxide, an alkali metal salt of a volatileweak acid or an ammonium salt of a volatile weak acid.

In the present specification, the term catholyte denotes the solution inthe cathode compartment and the term anolyte denotes the solution in theanode compartment.

According to a first way of carrying out the invention, a cell havingtwo compartments separated by a single anion exchange membrane is used.This first way will be denoted, in the text that follows, by (M and thetwo compartments will be denoted by (A) for the anode compartment, and(Ct) for the cathode compartment.

According to a second way of carrying out the invention, denoted in thetext which follows by (M at cell having three compartments separatedfrom one another by two anion exchange membranes is used; the threecompartments will be respectively denoted, going from the anode to thecathode, by (A), B and (Ct).

According to a third way of carrying out the invention, denoted in thetest that follows by (M a cell having four compartments separated fromone another by three anion exchange membranes is used; the fourcompartments will be respectively denoted, going from the anode to thecathode, by (A), (B (B and (Ct).

The nature of the solvent or solvents contained in the catholyte and theanolyte is not critical, it is usually preferred to use water.

The amount of thallous and thallic ions present in the anolyte ispreferably such that the number of gram atoms of thallium per litre ofsolution is: between 0.01 and 0.8, particularly between 0.1 to 0.5.

In order to improve the solubility of the thallic derivatives, it isoften advantageous to add an acid to the anolyte, for example perchloricacid, sulphuric acid, nitric acid, fiuoroboric acid, alkylsulphuricacids, or saturated aliphatic acids which are soluble in water, such asacetic acid. It is usually preferred, and especially in a sulphuric acidmedium, to operate at a pH below 2; with an aliphatic acid it ispreferred to operate at a pH less than 5.

In the process according to the invention the catholyte containing analkali metal or ammonium hydroxide or salt of a volatile weak acid; thealkali metal may be lithium, sodium, potassium, rubidium or caesium andthe ammonium compound may be quaternary or otherwise.

Ammonium derivatives which can be used are compounds comprising thegrouping of formula:

NH R

in which x and y are positive integers or are zero, the sum x-l-y isequal to 4, and R represents an organic radical, particularly alkyl, thegroup (I) preferably containing not more than 16 carbon atoms andpreferably being in the form of a salt of a volatile weak acid.Carbonates and bicarbonates are conveniently used as salts of a volatileweak acid.

The concentration of alkali metal or ammonium compound in the catholyteis generally between 0.1 and 5 mols/litre and preferably between 0.5 and2 mols/litre.

The anion exchange membranes separating the various compartments can behomogeneous membranes or heterogeneous membranes; the exchange groupscan, in particular, be ammonium, phosphonium or sulphonium groups, aswell as amine groups when the catholyte contains a salt of a volatileWeak acid. Ion exchange membranes of which the permeation selectivity(measured according to the technique described in French patentspecification No. 1,584,187) is greater than 40%, preferably greaterthan are preferably used.

The electric current densities in the process according to the inventionare generally between 0.5 and a./ dmfi, preferably between 5 and 25a./dm.

An electrically conducting material which is insoluble in the anolyteunder the operating conditions is used as used as the materialconstituting the anode. Suitable materials include graphite, platinum,lead dioxide, as well as lead and its alloys, particularly with silver,antimony and tin.

The cathode, may be of steel, particularly stainless steel, iron,copper, chromium, nickel, graphite, mercury or lead or its alloys,particularly with silver, antimony and tin.

In order to carry out the electrochemical oxidation according to theinvention, a simple electrolysis cell or an assembly of cells joinedtogether for example by the filterpress system, can be used.

The oxidation according to the invention can be carried out continuouslyor discontinuously. To operate continuously, the electrolyser is fedwith a solution of TH and simultaneously electrolysed solution iswithdrawn. The process of the invention can also be combined in one andthe same system with an oxidation process for organic compounds using Tlthe thallous-thallic solution continuously circulating alternately,first in the electrolysis cell, and then in the oxidisor for the organiccompound.

In all cases it is advantageous to establish a closed circuitcirculation of electrolyte in each of the compartments of theelectrolyser; in particular, this allows:

(a) The concentration of these electrolytes to be rendered uniform.

(b) These electrolytes to be cooled (the electrolysis gives rise toheating).

(c) The catholyte to be decanted externally if necessary, in the casewhere small quantities of thallium in suspension are formed.

The operating conditions are generally controlled so as to maintain anelectrolyte temperature which is neither below C. nor above 60 C. Theoxidation is preferably carried out at a temperature in the range 1535C.

In the course of the oxidation there is a migration of hydroxyl ionsfrom the catholyte towards the anolyte. These hydroxyl ions combine withprotons to form water; as a result there is a slight increase in volumeof anolyte and a reduction in its concentration, which can be easilycorrected, particularly in the case of operations carried out over along period of time or continuously, by a simple partial distillation ofthe anolyte.

When a salt of a volatile acid is used in the catholyte, there may occurliberation of a gas in the anolyte (CO in the case of a catholytecontaining carbonates). In order to stabilise the basicity of thecatholyte, it is advantageous to collect this gas which is liberated andto recycle it to the catholyte.

The various technical details which have been described above are validfor the three ways (M (M and (M In the following discussion, othertechnical details will be given, which relate more particularly to the(M or (M methods. The second, and if appropriate the third, ion exchangemembrane, should have the same characteristics as those indicatedpreviously for the membranes used in the (M process.

The liquid (also called electrolyte) used in the compartment of type (Bshould have the same characteristics as those defined previously for thecatholyte; it is preferred in practice that the composition of theelectrolyte (B be the same as that of the catholyte, except for the factthat the latter does not contain any thallous ions at all; theconcentration of the alkali metal or ammonium derivative in theelectrolyte (B can however be different to that of the catholyte.Moreover, the electrolyte (B is preferably subjected to a separateelectrolysis in an auxiliary electrolyser, which allows any traces ofthallium which may be present to be deposited in metal form; thisauxiliary electrolyser, which is of small dimensions in practice, isadvantageous provided with an anion exchange membrane separating thecatholyte and the anolyte, and the catholyte consists of liquid whichcomes from (B and the anolyte has a composition similar to that of thecatholyte except as regards thallium, which is absent in the anolyte ofthis auxiliary electrolyser.

The liquid (or electrolyte) present in the compartment of type (B is anaqueous solution containing thallous ions, thallic ions being absent oralmost absent (preferably less than 0.1% by weight, calculated as T1metal). This solution can also contain an acid; the proportion ofthallous ions and the nature and proportion of acid in this solution areselected in accordance with the same criteria as for the anolyte.

According to a first method for carrying out the (M process, thecompartments (B are fed with the thallous solution coming from a reactorusing thallic ions, and then the thallous solution leaving (B istransferred to the anode compartment (A), where it is subjected toanodic oxidation.

According to a second method of carrying out the (M process, thecompartments of type (A) and (B are simultaneously fed with a thalloussolution such as that coming from a reactor for the oxidation ofolefines by thallic ions, the solution present in the compartment (B iswithdrawn at a rate sufficient to maintain the proportion of thallicions Within the limit indicated above, and the said solution which hasbeen withdrawn from the compartment (B is transferred directly to thereactor for utilising the thallic ions; anolyte is withdrawnsimultaneously to transfer it also to the reactor for utilising thethallic ions.

In addition to the advantages already stated, the oxidation processaccording to the invention allows good electrical yields to be obtained.It also prevents any progressive concentration of non-thallous ornon-thallic salts or ions in the anolyte. The (M and (M methods ofcarrying out the process according to the invention moreover allow thedeposit of thallium on the cathode to be removed, this deposit having atendency to be formed during operations of a very long duration when thepermeation selectivity of the membranes is not close to 100%. Finally,the (M method allows the life of the membranes to be increased.

Thallic ions are useful as catalysts in the oxidation of organiccompounds e.g. olefines, as described in US. patent specification No.3,048,636 and in such processes are reduced to thallous ions. Theprocess of the present invention is useful in converting such thallousions back to thallic ions.

The following examples are given to illustrate the invention:

EXAMPLE 1 Tl+ is oxidised to Tl+++ in an electrolysis cell having thefollowing characteristics:

Anodez, lead, arranged horizontally, surface area 0.33

Cathode, copper, surface area 0.35 dm.

Catholyte, 100 cm. of aqueous 2.5 N sodium hydroxide solution.

Anolyte, 500 cm. of aqueous solution initially containing one mol/litreof sulphuric acid and 99 millimols/litre of thallous sulphate. Stirringis performed by means of a magnetic stirrer.

Anion exchange memberane, homogeneous memberane containing quaternaryammonium groups and based on a divinylbenzene-styrene copolymer(exchange capacity: 0.55 milliequivalent/g.; permeation selectivity:90%).

The cathode-membrane distance is 5 mm.

The anode-membrane distance is 10 mm.

The electric current passed is 5 a., the voltage is 8 v., and thetemperature is 27 C.

After 15,000 coulombs have passed, of the thalous ions have beenconverted into thallic ions; the electric current yield is 95.4%.

EXAMPLES 2 TO 6 A series of experiments involving the oxidation of Tl+to T1 is carried out.

The particular conditions relating to each example and the resultsobtained are given in Table I. l

The general conditions which are common to the various examples are asfollows:

The useful surfaces of the anode and cathode are equal to 2 dm. Thecathode is of stainless steel. The nature The expansion vessel is fed ata rate of about 25 cmfi/ minute.

The membrane has a permeation selectivity of 72%.

The anolyte initially contains 0.4 gram ions/litre of thalof the anodeis indicated in the table The membrane 5 lous and thallic ions in anumerical proportion of /90.

is one having quaternary ammonium groups and a permeation selectivityabove After the passage of 1,350,000 coulombs, the degree of Theelectrodes are separated from the membrane by a converslon 865%: and theyleld 15 955%- polypropylene fabric which is 2 mm. thick and has 10large meshes. EXAMPLE 9 A circulation of electrolyte is established ineach of the I compartments of the electrodialyser; the rate of movel 1SOXIdIScd to Ti in an electroly l Cell having ment over the electrodes is30 cm./seconds; the electhree compartments, the p ing OH it OIIS beingas trolyte circuits pass through a cooler, which allows the 15 follows:

temperature to be kept at 27 C.

In the Table I the letter M indicates a concentration The i f of lead.contamzmg 10% of antlmony; its of 1 mol/litre; in the case where thisletter is preceded usg u sur ace {area is 1 by a number this latterindicates the concentration ex- The Cathode havmg the same surface area15 of Stamless pressed in mols/litre. Steel The degree of convarsion isthe fraction (percentage The two membranes separating the threecompartments in numbers f ions) f converted into 13+ in the areheterogeneous membranes containing quaternary course f the experimentammonium groups; the matrix and the ion exchange The electric currentyield is the percentage of coulom-bs T6510 f in P P Q y Weight, of32/63; the which have served to oxidise 11+ to 13+ matrix 1s a vinylchloride/butyl maleate copolymer TABLE I Catholyte Anolyte ConcentrationDegree Electric Current of con cuirent Nature of the Concen- Volume,Volume, $12 894. passed Voltage Number of version, yield, Ex. anodeNature tration cm. cm. B04112 initial in a. 1n v. coulombs percentpercent 1 750 000 1 0.165 20 4. 30,000 71.5 01.5 2 iiEiL'IIIIIIIIIII$285 750 600 0.5 0.171 20 4.2 31,825 78 96.5 as 1 3- 193 a a s 0 N OH iii880 600 1 0.165 10 as 30,700 67.5 84.5

1 Concentration in mols/litre.

EXAMPLE 7 and 5 lead back from the expansion vessels 12 and 13 to theanode and cathode compartments. Expansion vessel 12 has inlet and outletports 10 and 11 for anolyte.

The electrolysis conditions are similar to those of Example 2, with thefollowing modifications:

The initial concentration of thallous ions (in the form of sulphate) is0.298 gram ions/litre.

The total volume of anolyte in circulation is 600 cmfi.

The total volume of catholyte in circulation is 1000 cm The ion exchangemembrane, which contains quaternary ammonium groups, has a permeationselectivity of 89% After the passage of 27,500 coulombs, a solutioncontaining 0.298 gram ions/litre of Tl+ (in the form of sulphate) and 1mol/ litre of H 80 is introduced via port 10, at the rate ofapproximately 26.1 cmfi/minute, to the anolyte; simultaneously the samequantity of electrolysed solution is withdrawn from the anolyte via port11.

After the passage of a total of 432,500 coulombs, thallic ions areobtained with a degree of conversion of 68% and with an electric currentyield of 84.5%.

EXAMPLE 8 Example 7 is repeated, with the following modifications:

The current passed is 27 a. The voltage is 5.1 v.

(96/4); the ion exchange resin is a resin containing quaternary ammoniumgroups and is based on a styrene-divinylbenzene copolymer. Thepermeation selectivity of the membrane is 78%, and its substitutionresistance is 15 ohms cmF.

The thickness of each compartment is 1 mm., and an interposedpolypropylene grill allows the membranes to be kept in place and theoutflow of the liquids to be suitably distributed.

The electrolytes which pass through the three compartments circulate inco-current, the pressure at the inlet being 2.3 bars absolute and at theoutlet being 1.1 bars.

The electrolyte of the central compartment (B passes,

in close circuit and in succession, through the compartment (B and thenthrough the cathode compartment of an auxiliary electrolytic cell havinga copper cathode of 5 cm. which functions under a current of 0.5 a.

The liquid passing through the central compartment and the catholyte areinitially an aqueous normal solution of sodium hydroxide.

A closed circuit circulation is established in the anolyte and in thecatholyte.

The anolyte consists initially of an aqueous solution containing 100g./l. of thallous sulphate and 20% by weight of H 50 it is fed at the:rate of 0.5 litre/hour by a solution having the same composition; theamount of anolyte is kept constant by a feed at a corresponding rate offlow.

The current passed is 10 a.

The temperature is 30 C.

The voltage is 6 v.

Electrolysis is carried out for 5 3 hours.

1924.6 g. of Tl+++ (expressed in T1 metal) are obtained, with anelectric current yield of Moreover, it is found that there is no depositof metallic thallium on the cathode of the main electrolyser, and themembranes are not deformed.

7 EXAMPLE 1o Tl+ is oxidised to Tl+++ in an electrolysis cell havingfour compartments, the operating conditions being as follows:

The anode is of lead and has a useful surface area of 1 The cathode isof stainless steel and has the same surface area.

The three membranes are identical to those of Example 9.

The thickness of the compartments is 1 mm., these compartments beingprovided with spacers as in Example 9.

The electrolytes passing through the four compartments circulate inco-current, the pressure at the inlet being 2.2 bars absolute, and atthe outlet being 1 bar absolute.

The catholyte and electrolyte contained in the compartment next to thecathode compartment consist initially of an aqueous normal solution ofsodium hydroxide.

The catholyte circulates in a closed circuit.

The electrolyte contained in compartment (B next to the cathodecompartment circulates and is subjected to an auxiliary electrolysis inthe same way as the electrolyte of the central compartment (B of Example9,

The anolyte consists initially of an aqueous solution containing 8.7% byweight of sulphuric acid and 7.02% by weight of TH (taken as Tl metal)in the form of Tl SO The electrolyte contained in the compartment (Bnext to the anode compartment has initially the same composition as theanolyte.

A closed circuit circulation is established in the anolyte,

and in the compartment (B The compartment (B next to the anodecompartment is ,fed continuously at the rate of 0.456 l./hur with athallous solution having the same composition as that given for theanolyte at the start of the electrolysis; withdrawal of liquid from thesame compartment allows the amount of liquid circulating therein to bekept constant, and serves to feed the anolyte.

A fraction of the anolyte is withdrawn so as to keep its volumeconstant.

The current passed is a.

The temperature is about 27 C.

The voltage is 6.4 v.

After the passage of 1,431,000 coulombs, 1312 g. of T1+++ calculated asT1 metal) are obtained, with an electric current yield of 86.6%.

It is found that there is no deposit of thallium on the cathode of themain electrolyser, and the mechanical and electrochemical properties ofthe membranes have not altered.

We claim:

1. A process for the electrochemical oxidation of thallic ions whichcomprises passing a direct electric current between an anode and acathode of an electrolysis cell divided into at least two compartmentsby at least one membrane having anion exchange properties, the anolytecomprising thallous and thallic ions and the catholyte comprising acompound M+A- where M+ represents an alkali metal or ammonium ion and A-represents a hydroxide, carbonate or hydrogen carbonate ion.

2. A process according to claim 1, wherein a liquid circulation isestablished in each of the compartments.

3. A process according to claim 1, wherein the electrolysis cell isdivided into two compartments by means of one anion exchange membrane.

4. A process according to claim 1, wherein the electrolysis cell isdivided into three compartments by means of two anion exchangemembranes.

5. A process according to claim 1, wherein the electrolysis cell isdivided into four compartments by means of three anion exchangemembranes.

6. A process according to claim 4, wherein the composition of the liquidcontained in the compartment next to the cathode compartment isessentially the same as that of the catholyte.

7. A process according to claim 6, wherein the liquid in the compartmentnext to the cathode compartment is subjected to electrolysis in anauxiliary electrolyser to convert any thallium present into the metallicform.

8. A process according to claim 5, wherein the composition of the liquidcontained in the compartment next to the cathode compartment isessentially the same as that of the catholyte.

9. A process according to claim 8, wherein the liquid in the compartmentnext to the cathode compartment is subjected to electrolysis in anauxiliary electrolyser to convert any thallium present into the metallicform.

10. A process according to claim 5, wherein the liquid contained in thecompartment next to the anode compartment is an aqueous solutioncontaining thallous ions.

11. A process according to claim 1 wherein the cathode is stainless andthe anode is lead, lead oxide or a lead/ silver or lead/antimony alloy,the anolyte contains sulphuric acid and the catholyte comprises anaqueous solution of sodium hydroxide or sodium hydrogen carbonate.

12. A process according to claim 11 wherein the cell has from 2 to 4compartments separated from one another by from 1 to 3 membranesrespectively, having quaternary ammonium groups, electrolyte iscirculated in each compartment, the temperature of the electrolyte ismaintained at about 30 C. and the current density is about 5-25amperes/sq. decimetre.

13. A process according to claim 1 wherein the concentration of the saltin the catholyte is 0.1-5.0 moles/ litre.

14. A process according to claim 1 wherein the current density is 0.5-amp./dm.

15. A process according to claim 1 wherein the electrolyte temperatureis 0 C. to 60 C.

16. A process according to claim 1 wherein the alkali metal is lithium,sodium, potassium, rubidium or caesium and the ammonium ion is offormula NH R where x and y are independently 0 or a positive integer,the sum x+y being equal to 4, and R represents an alkyl radical thegroup R containing up to 16 carbon atoms.

References Cited UNITED STATES PATENTS 3,479,262 11/1969 MacLeau et a1.204-80 3,616,276 10/1971 Schneder et al. 204-86 X 3,057,794 10/ 1962Carlin 204-252 3,113,911 12/1963 Jones 20494 3,486,992 12/ 1969 Frye204--86 FOREIGN PATENTS 73,069 8/1943 Bohemia and Moravia FREDERICK C.EDMUNDSON, Primary Examiner US. Cl. X.R. 20486, 91, R

V UNITED STATES ATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 r r804 Dated September 18 1-973 I Inventor(s) LOUIS LE BRIS t al It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

m In the heading:

Please correct the claim for Convention priority v .as follows; I t

-'--Claims priority, applications F rance' t February 17, 1970, No.70,05623 and v 'December 31, 1970, No. 7o,47s48-- I y I Signed aridsealed this 30th day of July 1974.

v L) v r Attest:

MeCOY M. GIBSON, JR. C. MARSHALL DANN Attesting Officer CommissionerofPatents USCOMM'DC 60316-969 FORM Po-wsq no-s9).

